Electric lamps known as double enveloped lamps include a light-source capsule, or arc tube, and an outer envelope surrounding the light source capsule. Metal halide arc lamps are examples of double-enveloped lamps. In such double enveloped lamps, there is a small probability that the light source capsule will burst. When such an event occurs, hot fragments of lass, or shards, and other capsule parts emanating from the burst capsule are forcibly propelled against the outer envelope. If the outer envelope also shatters, there is a potential safety hazard to persons or property in the immediate surroundings. Failure of the outer envelope is known as a containment failure.
One way to avoid the safety hazard of containment failure is to operate the lamp within a protective fixture that is capable of containing such a failure. However, a protective fixture usually incurs additional cost, particularly when an existing fixture must be modified or replaced. Furthermore, a protective fixture reduces the light output of the lamp, and it may be more difficult and expensive to replace the lamp in a protective fixture.
A preferred solution to the containment failure problem is a lamp assembly capable of self containment. One known self containment technique is to make the outer envelope sufficiently strong to contain the shattered light source capsule. An outer envelope having a relatively thick wall in combination with a light source capsule having a relatively thin wall is disclosed in U.S. Pat. No. 4,598,225 issued Jul. 1, 1986 to Gagnon. Another prior art technique is to shield the outer envelope from the effects of a burst light-source capsule. In U.S. Pat. No. 4,580,989 issued Apr. 8, 1986 to Fohl et al, a light-transmissive enclosure, or shield, located within an outer envelope surrounds a light source capsule and shields the outer envelope. See also U.S. Pat. No. 4,281,274 issued Jul. 28, 1981 to Bechard et al. Still another technique for containment is to reinforce the shield or the light-source capsule. In U.S. Pat. No. 4,721,876 issued Jan. 26, 1988 to White et al, a light- transmissive shield is reinforced by a cloth like wire mesh. Wire mesh reinforcement of a light-source capsule is disclosed in U.S. Pat. No. 4,625,140 issued Nov. 25, 1986 to Gagnon. Containment techniques are also disclosed in U.S. Pat. No. 4,888,517 issued Dec. 19, 1989, U.S. Pat. No. 4,942,330 issued Jul. 17, 1990 and in pending application Ser. No. 07/468,042 filed Jan. 20, 1990, all assigned to the assignee of the present application.
Guards for protecting incandescent lamps are disclosed in U.S. Pat. Nos. 314,208 issued Mar. 18, 1885 to White, 765,568 issued Jul. 19, 1904 to Eisenmann and 781,391 issued Jan. 31, 1905 to Blake.
While the above referenced containment techniques are highly effective for some lamp types and sizes, they may have disadvantages when applied to other lamp types and sizes. For example, the use of a thick walled outer envelope is effective for relatively small lamps but adds to the cost of the lamp. Lamps of greater than about 400 watts having a thick walled outer envelope are so heavy that there is a possibility that the lamp base will fail, leading to the lamp falling out of the light fixture. Furthermore, thick-walled outer envelopes of large physical size are difficult to fabricate. Various wire mesh containment devices are effective in achieving containment but add to the cost of the lamp and block a portion of its light output. Quartz shields, or shrouds, between the arc tube and the outer envelope are also effective in achieving containment. However, the shroud disturbs the thermal environment of the arc tube. Thus, the arc tube and the structures surrounding it must be carefully designed for proper lamp operation.
It is a general object of the present invention to provide improved double-enveloped lamps.
It is another object of the present invention to provide improved double enveloped lamps which can be safely operated without a protective fixture.
It is a further object of the present invention to provide self-containing, double enveloped lamps which have a high luminous output.
It is still another object of the present invention to provide improved metal halide arc discharge lamps.
It is a further object of the present invention to provide self-containing, double enveloped lamps which are light in weight and low in cost.